Modulating Performance and Stability of Inorganic Lead-Free Perovskite Solar Cells via Lewis-Pair Mediation

Abstract

Fully inorganic perovskites based on Bi³⁺ and Sb³⁺ are emerging as alternatives that overcome the toxicity and low stability of their Pb-based perovskite counterparts. Nevertheless, the thin film fabrication of Pb-free perovskites remains a struggle, with poor morphologies and incomplete conversions greatly inhibiting device performance. In this study, we modulated the crystallization of an all-inorganic dimer phase of a Sb perovskite (d-Cs₃Sb₂I₉) through gradual increase in the annealing temperature, accompanied by the use of Lewis bases for adduct formation. Here, the role of Lewis pairing in the crystallization of the resulting Pb-free Cs₃Sb₂I₉ thin films has been investigated. Both, "S-donor" (thiourea) and "O-donor" [<i>N</i>-methylpyrrolidone (NMP)] Lewis bases are examined for their abilities to form adducts with Cs⁺ and Sb³⁺ cations. Furthermore, density functional theory has been used to estimate the binding energies of these Lewis bases with the Cs₃Sb₂I₉ lattice. Temperature-dependent photoluminescence spectroscopy revealed the nature of the band gap of d-Cs₃Sb₂I₉. The efficiency of the resulting perovskite solar cells was enhanced to 1.8%, with excellent stability observed, when using NMP to form the adduct film. To the best of our knowledge, this is the best solar cell efficiency for the dimer phase of the inorganic Sb-based perovskite. The effects of both S- and O-donors are studied under various environmental stresses to reveal the stability responses of the devices.